This 50mm reaches its best DxOMark low-light score at f/2.8 on both APS-C and full-frame sensors, and has to be stopped down to f/4 to achieve its best mid-light score.
This makes this 50mm a suitable candidate for portraits or reportage. Distortion is handled well, as well as chromatic aberrations are on full-frame; these perturbations are slightly more noticeable on the edges when the lens is attached to a D300s, for instance. The biggest concern with this very fast lens is that it is hardly usable at its maximum aperture and has to be stopped down by 2 stops to reach a good enough definition.

On a full-frame sensor (D3X):

When installed on a D3x, the first noticeable detail is that opening the lens wide results in strongly deteriorated resolution, between 20 and 30lp/mm on the whole frame. The central first one-third becomes interesting when the lens is stopped down to f/2.8, and becomes very sharp at f/4and f/5.6. But the edges remain much softer. To improve them, one must stop down to f/11, thus lowering the resolution in the middle of the field. Visible at f/1.4, vignetting disappears as soon as the lens is stopped down to f/2.8.

On an APS-C sensor (D300s):

On a smaller-sized sensor, the resolution behaves the same way: very poor when the lens is wide open, it improves considerably when stopping down to f/4 and f/5.6, but the last one-third of the field remains very soft. At full aperture, vignetting is present, although quite light; 1 stop later, at f/2, it has completely disappeared.

Further readings for the Latest results for Zeiss lenses

To provide photographers with a broader perspective about mobiles, lenses and cameras, here are links to articles, reviews, and analyses of photographic equipment produced by DxOMark, renown websites, magazines or blogs.

Carl Zeiss is one of those almost mythical lens makers. Focusing on quality, its lenses almost always rely on complex optical formulas. Their price reflects this involvement in quality, and logically Zeiss lenses are on the same shelves as the very high-end Canon or Nikon models. We have tested 14 of them so far, ranging from the ultra-wide-angle Distagon T18mm f/3.5 to the much heavier Makro Planar T100mm f/2.